1. Field of the Invention
The present invention relates to semiconductor devices each having a circuit including a thin film transistor (hereinafter referred to as a TFT) and manufacturing methods thereof. For example, the present invention relates to electronic appliances in each of which an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element is mounted as a component.
Note that semiconductor devices in this specification refer to general devices which can function by utilization of semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic appliances are all semiconductor devices.
2. Description of the Related Art
Various types of memories have conventionally been proposed. As typical memories, the following can be given: memories each including an electromagnetic tape or an electromagnetic disk, a RAM capable of writing and reading, a ROM only for reading (read only memory), and the like.
As conventional ROMs, the following can be given: a mask ROM which stores data with a mask in an IC manufacturing process, a fuse-type ROM which stores data by melting down a fuse element with current after manufacture of an IC chip, an antifuse-type ROM which stores data by short-circuiting an insulator with current after manufacture of an IC chip, and the like.
Since a mask ROM stores data with a mask in an IC manufacturing process, a mask corresponding to data to be written has to be prepared; thus, the manufacturing cost has been increased. In addition, a fuse-type ROM could cause malfunction due to dust generated when a fuse element is melted down.
Further, an antifuse-type ROM is more advantageous than other ROMs in that a mask corresponding to data to be written at the time of manufacturing is not necessary and dust is not generated when data is written to the memory. As an example of an antifuse-type ROM formed on a silicon substrate, technology described in Patent Document 1: Japanese Published Patent Application No. H7-297293 is given.
FIG. 13 shows a cross-sectional view of an antifuse-type ROM disclosed in Patent Document 1. In FIG. 13, a silicon substrate 50 over which an nMOS transistor is formed, an amorphous silicon film 53, a tungsten film 54, a tungsten film 54′, and an Al—Si—Cu wiring 55 are formed. Although reference numerals 51 and 52 are not clearly identified in Patent Document 1, the reference numeral 51 is probably an n+ drain region and the reference numeral 52 is probably a SiO2 film formed by a CVD method. Patent Document 1 is characterized in that the tungsten film 54′, the amorphous silicon film 53, and the tungsten film 54 which form a stacked film are continuously formed with a multi-chamber system without exposure to the air.
In addition, an antifuse-type ROM using a main body of germanium by being in contact with a main body containing aluminum is disclosed in Patent Document 2: Japanese Published Patent Application No. H6-260558.
Moreover, an antifuse-type ROM in which an amorphous silicon film containing germanium is interposed between wirings is disclosed in Patent Document 3: Japanese Published Patent Application No. H5-343633.
In recent years, a semiconductor device with a wireless communication function, specifically a wireless chip, has been expected to have a large market, thereby having attracted attention. Such a wireless chip may be referred to as an ID tag, an IC tag, an IC chip, a radio frequency (RF) tag, a wireless tag, an electronic tag, and a radio frequency identification (RFID) in accordance with usage.
A wireless chip includes an interface, memory, a control portion, and the like. As the memory, a RAM capable of writing and reading and a ROM only for reading are used, and they are separately used in accordance with purposes. Specifically, a memory region is assigned for each specified application, and an access right is managed for each application and each directory. In order to manage the access right, the wireless chip has a verification unit which compares and verifies a private code of application and a control unit which gives users the access right regarding the application to which the private code is identical, according to the comparison and verification by the verification unit. Such a wireless chip is formed using a silicon wafer, and integrated circuits such as a memory circuit and an arithmetic circuit are integrated on a semiconductor substrate.
When a card to which such a wireless chip is mounted (a so-called IC card) is compared with a magnetic card, the IC card has advantages of having a large memory capacity, an arithmetic function, high authentication accuracy, and great difficulty in falsification. Therefore, the IC card is suitable for management of personal information. A ROM only for reading is commonly used as a memory mounted to an IC card so that falsification is impossible.